1. Field of the Invention
The present invention relates to a light guide, a line-illuminating device, and a contact-type or reduction-type image-scanning device (image sensor) in which the line-illuminating device is incorporated.
2. Description of the Prior Art
An image-scanning device for a facsimile machine, a copying machine, an image scanner or the like is provided with a line-illuminating device for linearly illuminating a document surface along a main scanning field. In the line-illuminating device, light is introduced into an end surface of a bar-shaped or plate-shaped transparent light guide, reflected on the internal surfaces, and emitted from a light-emitting surface.
Japanese Patent Application Publication No. 8-43633 has disclosed a light guide having a cylindrical shape in which incident light from the end surface is reflected on a light-reflecting portion provided on the bottom surface so as to be emitted from the light-emitting surface.
Japanese Patent Application Publication No. 10-112782 has disclosed a light guide having a plate shape in which incident light from the end surface is reflected on a light-reflecting portion provided on the bottom surface so as to be emitted from the light-emitting surface. According to paragraph [0009] of this document, the light-reflecting portion includes a plurality of tubular concave portions whose axis is perpendicular to the longitudinal direction of the light guide.
Japanese Patent Application Publication No. 2001-201639 has disclosed that a light scattering pattern is formed with laser light, and that light-reflecting portions near to the end surface are spaced apart in an island shape. This document has also disclosed that a plurality of light scattering patterns may be superposed in paragraph [0016] and FIG. 4 (b).
Japanese Patent Application Publication No. 2004-56425 has disclosed that flat portions are formed between the bottom surface and both side surfaces of a light guide by C-chamfering in paragraph [0006] and FIG. 8 (a).
Japanese Patent Application Publication No. 8-163320 has proposed that a light scattering pattern is formed on a surface of a light guide by printing white pigment, and the width of the light scattering pattern is gradually decreased toward a light-emitting unit. Also, Japanese Patent Application Publication No. 10-126581 has proposed that a light scattering pattern formed on the end near to a light-emitting unit is a discontinuous shape.
Japanese Patent Application Publication No. 4-329521 has disclosed a light-reflecting portion including fine spherical concave surfaces, and the spherical concave surface is a small concave portion having a hemispherical shape in paragraph [0034].
Japanese Patent Application Publication No. 11-313195 has disclosed that both side surfaces of a light guide are a curved surface whose curvature is increased toward the bottom surface. Japanese Patent Application Publication No. 2002-190917 has disclosed that a light guide is accommodated into a case provided with a light-emitting unit in a state where a light-emitting surface is exposed. Japanese Patent Application Publication No. 11-284232 has disclosed that a light-emitting unit is directly attached to an end surface of a light guide. Japanese Patent Application Publication No. 2000-307808 has disclosed that a light-emitting unit includes light-emitting elements of three primary colors, i.e., Red, Green, and Blue, and the light-emitting elements are positioned at the same interval from a point of the bottom surface of the light guide. Japanese Patent Application Publication No. 11-136451 has disclosed that a light-emitting unit includes light-emitting elements of three primary colors, i.e., Red, Green, and Blue, and the light-emitting elements are positioned at the same interval from the bottom surface of the light guide.
Among the above-mentioned prior art materials, Japanese Patent Application Publication No. 10-112782, Japanese Patent Application Publication No. 2001-201639 and Japanese Patent Application Publication No. 4-329521 have disclosed a light-reflecting portion including a plurality of concave portions. Specifically, the light scattering pattern of Japanese Patent Application Publication No. 2001-201639 is formed by ablation (evaporation) which occurs by irradiating part of the light guide with laser light so that the energy of the laser light is absorbed. However, since the surface of the light scattering pattern is damaged by the ablation, it is impossible to adjust the direction of light scattered by the light scattering pattern.
Japanese Patent Application Publication No. 10-112782 has disclosed a concave portion having a half-cylinder shape, and Japanese Patent Application Publication No. 4-329521 has disclosed a concave portion having a hemispherical shape. Seen from the side of the light guide (the cross section along the longitudinal direction), the concave portion having a half-cylinder shape and the concave portion having a hemispherical shape has the same shape. Thus, light scattering only in the case of the concave portion having a half-cylinder shape will be explained with reference to FIG. 16.
As shown in FIG. 16, among a plurality of concave portions having a half-cylinder shape, as for the concave portion nearest to the light source, light is incident on the base portion. Since the angle of the base portion of the concave portion is close to 90°, the light returns toward the light source side as shown in the figure. On the other hand, as for the other concave portions, since light is incident only on the area near to the bottom surface of the concave portion, the reflected light never returns toward the light source side so that the light can be diffused in a substantially uniform manner. Consequently, when a plurality of concave portions having a half-cylinder shape are formed, only the concave portion nearest to the light source reflects light differently, which results in a lack of uniformity of light distribution.